Network Working Group T. Kindberg
Request for Comments: 4151 Hewlett-Packard Corporation
Category: Informational S. Hawke
World Wide Web Consortium
October 2005
The 'tag' URI Scheme
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Disclaimer
The views and opinions of authors expressed herein do not necessarily
state or reflect those of the World Wide Web Consortium, and may not
be used for advertising or product endorsement purposes. This
proposal has not undergone technical review within the Consortium and
must not be construed as a Consortium recommendation.
Abstract
This document describes the "tag" Uniform Resource Identifier (URI)
scheme. Tag URIs (also known as "tags") are designed to be unique
across space and time while being tractable to humans. They are
distinct from most other URIs in that they have no authoritative
resolution mechanism. A tag may be used purely as an entity
identifier. Furthermore, using tags has some advantages over the
common practice of using "http" URIs as identifiers for
non-HTTP-accessible resources.
Kindberg & Hawke Informational [Page 1]RFC 4151 Tag URIs October 2005Table of Contents
1. Introduction ....................................................2
1.1. Terminology ................................................3
1.2. Further Information and Discussion of this Document ........4
2. Tag Syntax and Rules ............................................4
2.1. Tag Syntax and Examples ....................................4
2.2. Rules for Minting Tags .....................................5
2.3. Resolution of Tags .........................................7
2.4. Equality of Tags ...........................................7
3. Security Considerations .........................................7
4. IANA Considerations .............................................8
5. References ......................................................9
5.1. Normative References .......................................9
5.2. Informative References .....................................9
1. Introduction
A tag is a type of Uniform Resource Identifier (URI) [1] designed to
meet the following requirements:
1. Identifiers are likely to be unique across space and time, and
come from a practically inexhaustible supply.
2. Identifiers are relatively convenient for humans to mint
(create), read, type, remember etc.
3. No central registration is necessary, at least for holders of
domain names or email addresses; and there is negligible cost to
mint each new identifier.
4. The identifiers are independent of any particular resolution
scheme.
For example, the above requirements may apply in the case of a user
who wants to place identifiers on their documents:
a. The user wants to be reasonably sure that the identifier is
unique. Global uniqueness is valuable because it prevents
identifiers from becoming unintentionally ambiguous.
b. The identifiers should be tractable to the user, who should, for
example, be able to mint new identifiers conveniently, to
memorise them, and to type them into emails and forms.
c. The user does not want to have to communicate with anyone else in
order to mint identifiers for their documents.
Kindberg & Hawke Informational [Page 2]RFC 4151 Tag URIs October 2005
d. The user wants to avoid identifiers that might be taken to imply
the existence of an electronic resource accessible via a default
resolution mechanism, when no such electronic resource exists.
Existing identification schemes satisfy some, but not all, of the
requirements above. For example:
UUIDs [5], [6] are hard for humans to read.
OIDs [7], [8] and Digital Object Identifiers [9] require entities to
register as naming authorities, even in cases where the entity
already holds a domain name registration.
URLs (in particular, "http" URLs) are sometimes used as identifiers
that satisfy most of the above requirements. Many users and